I like most English accents. Kate Winslet has a very sexy voice, for example.
But I think the sexiest voice has to be a posh Dutch girl's voice (from the best of school unions, obviously) that is hoarse from smoking and drinking whiskey.
I always thought that singer Donna Lewis had an unbelievably sexy voice. Just listen to her sing "I Love You Always Forever" and tell me it doesn't play your spine like a harp.